Cellulose and hemicellulose are the most abundant plant materials produced by photosynthesis. They can be degraded and used as an energy source by numerous microorganisms, including bacteria, yeast and fungi, which produce extracellular enzymes capable of hydrolysis of the polymeric substrates to monomeric sugars (Aro et al., 2001). As the limits of non-renewable resources approach, the potential of cellulose to become a major renewable energy resource is enormous (Krishna et al., 2001). The effective utilization of cellulose through biological processes is one approach to overcoming the shortage of foods, feeds, and fuels (Ohmiya et al., 1997).
Cellulases are enzymes that hydrolyze cellulose (beta-1,4-glucan or beta D-glucosidic linkages) resulting in the formation of glucose, cellobiose, cellooligosaccharides, and the like. Cellulases have been traditionally divided into three major classes: endoglucanases (EC 3.2.1.4) (“EG”), exoglucanases or cellobiohydrolases (EC 3.2.1.91) (“CBH”) and beta-glucosidases ([beta]-D-glucoside glucohydrolase; EC 3.2.1.21) (“BG”) (Knowles et al., 1987 and Schulein, 1988). Endoglucanases act mainly on the amorphous parts of the cellulose fiber, whereas cellobiohydrolases are also able to degrade crystalline cellulose.
Cellulases are known to be produced by a large number of bacteria, yeast and fungi. Certain fungi produce a complete cellulase system capable of degrading crystalline forms of cellulose, such that the cellulases are readily produced in large quantities via fermentation.
In order to efficiently convert crystalline cellulose to glucose the complete cellulase system comprising components from each of the CBH, EG and BG classifications is required, with isolated components less effective in hydrolyzing crystalline cellulose (Filho et al., 1996). In particular, the combination of EG-type cellulases and CBH-type cellulases interact to more efficiently degrade cellulose than either enzyme used alone (Wood, 1985; Baker et al., 1994; and Nieves et al., 1995).
Additionally, cellulases are known in the art to be useful in the treatment of textiles for the purposes of enhancing the cleaning ability of detergent compositions, for use as a softening agent, for improving the feel and appearance of cotton fabrics, and the like (Kumar et al., 1997). Cellulase-containing detergent compositions with improved cleaning performance (U.S. Pat. No. 4,435,307; GB App. Nos. 2,095,275 and 2,094,826) and for use in the treatment of fabric to improve the feel and appearance of the textile (U.S. Pat. Nos. 5,648,263, 5,691,178, and 5,776,757, and GB App. No. 1,358,599), have been described in the literature.
Hence, cellulases produced in fungi and bacteria have received significant attention. In particular, fermentation of Trichoderma spp. (e.g., Trichoderma longibrachiatum or Trichoderma reesei) has been shown to produce a complete cellulase system capable of degrading crystalline forms of cellulose. Over the years, Trichoderma cellulase production has been improved by classical mutagenesis, screening, selection and development of highly refined, large scale inexpensive fermentation conditions. While the multi-component cellulase system of Trichoderma spp. is able to hydrolyze cellulose to glucose, there are cellulases from other microorganisms, particularly bacterial strains, with different properties for efficient cellulose hydrolysis, and it would be advantageous to express these proteins in a filamentous fungus for industrial scale cellulase production. However, the results of many studies demonstrate that the yield of bacterial enzymes from filamentous fungi is low (Jeeves et al., 1991).
In this invention, a heterologous exo-endo cellulase fusion construct, which includes the coding region of a fungal exo-cellobiohydrolase (CBH) catalytic domain and a coding region of an endoglucanase (EG) catalytic domain, has been introduced and expressed in a filamentous fungi host cell to increase the yield and effectiveness of cellulase enzymes.